Production of Near Zero Aromatics Containing Diesels

ABSTRACT

The invention provides a process for the production of synthetically derived diesel and diesel produced by the process. The process includes the steps of catalytic conversion of Fisher-Tropsch derived light olefins to distillates (COD) over a zeolite type catalyst at pressures of more than 50 barg, one step hydrotreating the COD product to simultaneously hydrogenate both olefins and aromatics, and collecting a hydrotreated fraction boiling between about 180° C. to 360° C.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a process for the production of synthetically derived near zero aromatic content diesel.

BACKGROUND TO THE INVENTION

It is well known that aromatics are carcinogenic and undesired in diesels. Ever evolving vehicle emission control regulations require low aromatic content diesel fuels.

Crude derived diesels are not popular due to their high sulphur content.

While High Temperature Synthetically derived Fischer-Tropsch (FT) diesel fuels have a low or near zero sulphur content, they are associated with relatively high aromatic levels some of which are multiple ring aromatic compounds which are considered undesirable.

It is an object of this invention to provide a one step process for the production of a synthetically derived diesel having typically less than 0.1% of mono cyclic (single ring) aromatics and no hazardous multiple ring aromatics (polycyclics). It is also an object of this invention to provide near zero aromatic diesel with good cold flow properties and low emission qualities.

Normally catalytical conversion of Fisher-Tropsch derived olefins with shape selective zeolites to distillates (COD) produce distillates having more than about 10% aromatics.

In this specification, references to percentage proportions refer to volume percentage proportions unless otherwise specified.

Traditionally hydrogenation of synthetically derived distillates is performed in two steps i.e. olefin hydrogenation followed by aromatic hydrogenation using a second more active catalyst. Catalysts used for the first step normally include cobalt and/or other metals. These catalysts normally need to be sulphided with sulphur containing compounds such as DMDS to ensure that the catalyst remains in the sulphided form. The sulphur containing compounds also quench the activity of the catalyst preventing runaway reactions emanating from highly reactive olefinnic feeds.

GENERAL DESCRIPTION OF THE INVENTION

According to a first aspect of the invention, there is provided a process for the production of synthetically derived diesel, which process includes the steps of:

catalytic conversion of Fisher-Tropsch derived light olefins to distillates (COD) over a zeolite type catalyst at pressures of more than 50 barg;

one step hydrotreating the COD product to simultaneously hydrogenate both olefins and aromatics; and

collecting a hydrotreated fraction boiling between about 180° C. to 360° C.

The Fisher-Tropsch derived olefins are converted to distillates over a shape selective zeolite type catalyst, a COD-9 or similar catalyst as defined by the International Zeolite Association (IZA). More specifically the Fisher-Tropsch derived light olefins comprising of carbon numbers 3 to 6 are oligomerised and isomerised to produce an intermediate highly branched olefinnic distillate.

The catalytic conversion reactor temperature may be maintained below 280° C.

The one step deep hydrotreating step may include hydrogenation over a Group 10 metal catalyst.

The Group 10 metal catalyst may include a high nickel content.

Alternatively, the Group 10 catalyst may include a noble metal catalyst such as supported platinum catalysts. These catalysts may also be bimetallic.

The support for the metal may be neutral. The applicant is aware that an acidic support causes unwanted cracking during hydrogenation.

The catalyst may be Nickel supported on alumina or platinum supported on allumina. (Sud Chemie G134 or Axens LD 402).

The one step hydrotreating step may include hydrogenation over a high nickel content hydrotreating catalyst or hydrotreating with a noble metal catalyst. Reactor pressures for such reactions would typically range from 5000 kPa to about 8000 kPa but not excluding higher pressures. Reaction temperatures vary from about 200° C. to 260° C. while the LHSV range from 0.3 to 2 depending on the feed.

The olefin content measured as Bromine Number determines the reactivity of a particular feed, highly reactive feeds may require a portion of the hydrogenated product to be recycled to quench the hydrogenation reaction. The LHSV may also be altered to below 0.5 to control excessive exothermic reactions.

The catalyst may be loaded into the reactor bed in an increased graded approach to limit an excessive exothermic reaction developing at the top of the reactor. The catalyst bed may have multiple zones with increased grades. Typically, a 4-zone graded catalyst bed. The concentration of the active catalyst in each of the 4 zones may be diluted with an inert ceramic in the following typical ratios of catalyst to ceramics, 0.2; 0.5; 170.0 and 650.

The Fisher-Tropsch derived olefins are converted to distillates over a shape selective zeolite catalyst. The conversion includes oligomerising and isomerising of the Fisher-Tropsch derived olefins to produce an intermediate olefinic COD product.

The catalytic conversion at pressures of more than 50 barg and/or a reactor temperature maintained below 280° C. produces a product stream with low aromatics and it will be appreciated that the relative low aromatics from the COD step allows moderate hydrogenation reactor conditions.

The process may include the step of blending the intermediate COD product or the hydrotreated fraction with crude derived diesels to enhance performance and/or emission properties of crude derived diesels.

The process may include the step of blending the intermediate COD product or the hydrotreated fraction with biodiesel derived from vegetable oils to improve the cold flow properties and motoring performance of the biodiesel.

The process may include the step of blending the intermediate COD product or the hydrotreated fraction with alcohols to reduce particulate matter emissions from fuels derived from intermediate COD product or the hydrotreated fraction. The alcohols may range from 1 to 5 carbon alcohols, preferably 2 to 5 carbon alcohols.

According to a second aspect of the invention there is provided a synthetically derived diesel produced by a catalytic conversion of Fisher-Tropsch derived olefins to distillates (COD) and deep hydrotreating thereof, the diesel boiling in the range of about 180 to 360° C., and including:

less than 10% n-paraffins;

more than 60% iso-paraffins; and

less than 1% aromatics.

The iso-paraffins may be predominantly methyl branched.

The methyl branched iso-paraffins may range between 50 and 80%, preferably between 60 and 70%, of the iso-paraffins.

The diesel may include about 20% naphtenes.

The naphtenes may be predominantly monocyclics.

The applicant has found that a relatively high cetane number can be maintained by replacing n-paraffins with predominantly methyl branched iso-paraffins and monocyclic naphtenes instead of more branched iso-paraffins and bicyclic naphtenes.

The boiling range may be between 180 and 360° C., however preferably between 210 and 345° C.

The flash point of the diesel as measured by ASTM D93 may be higher than 60° C. but preferably as high as 100° C. significantly improving the safety of product during handling.

The diesel fuels kinematic viscosity at 40° C. as measured by ASTM D445 may be below about 5.0 cSt but not lower than 2.0 cSt as measured at 40° C. The kinematic viscosity plays a role in the diesel fuel pump ability as well as the fuel injectors ability to efficiently inject fuel. High viscosity fuels negatively influence the fuel atomisation process limiting the formation fine droplets that lead to poor air fuel mixing within the combustion chamber (cylinder) resulting in turn in incomplete combustion accompanied by loss of power and economy. Excessively low viscosities lead to fuel pump leakage, incorrect metering and the inability for the fine atomised spray to penetrate the length of the combustion chamber and will result in poor combustion and in turn, result in loss of power and economy. A viscosity between 2.2 and 2.8 cSt as tested by ASTM D445 at 40° C. is preferred.

The cold flow properties are excellent due to the over 800 isoparaffinic hydrocarbon molecules that the diesel fuel comprises of. Low temperature operability is measured by Cold Filter Plugging Point (CFPP) as tested by IP 309 and is as low as minus 45° C. enabling these fuels to be used even in extreme low temperature conditions.

The total sulphur content of the diesel may be below 2 ppm (m/m) as measured by ASTM 3120. Sulphur in diesels creates a distinctive odour and contributes to the emission of particulate matter during combustion. Fuels with such low sulphurs enable modern vehicle exhaust emission treatment technologies.

The olefins content may be respectively reflected by a Bromine Number of well below 1 mg/100 g as measured by IP 129 and a peroxide number of less than 1 mg/100 g as measured by ASTM D3703.

The diesel may include blends with crude derived diesels in 10 to 90% v/v, preferably 30 to 70% v/v ratios.

The diesel may include blends with biodiesel derived from vegetable oils.

The diesel may include blends with 1 to 5 carbon alcohols in 0.5 to 10% v/v,

The compositions of the fuels are such that once combusted they offer excellent environmental benefits over crude derived diesel fuels. Exhaust emission qualities as tested on heavy-duty vehicles and compared against a standard US pump crude derived diesel fuel offered reductions in the following regulated emissions; particulate matter, oxides of nitrogen, carbon monoxide, carbon dioxide and hydrocarbons.

These fuels offer a relatively high Cetane Number of greater than 50 as tested by ASTM D613.

DETAILED DESCRIPTION OF THE INVENTION

The invention is now further described by non limiting examples.

Example 1

Light olefins in the carbon range C3 to C6 originating from a the High Temperature Fischer Tropsch plant located in Mossel Bay were oligomerised over a proprietary zeolyte catalyst (COD 9) as supplied by Sud Chemie. The oligomerisation reaction was performed at moderate temperatures below 280° C. and relatively high pressures of 55 bar process were used for the oligomerisation reaction to produce an intermediate olefinic distillate with a Bromine Number of over 90 g Br/100 g sample and containing 5.8% aromatics. Normally the intermediate distillate would contain more than 10% aromatics. This distillate was hydrotreated in one step using a high Nickel content commercial catalyst as supplied by Sud Chemie (Sud Chemie G134). The catalysts (about 270 cc) were loaded into a pilot plant reactor in a graded bed format and diluted with inert ceramics in the ratios of catalyst to ceramics of, 0.2; 0.5; 170.0 and 650. The reactor pressure was maintained at 58 bar, the WABT did not exceed 220° C., the LHSV was maintained at 0.9 and a third of the product was recycled back to the feed.

The one step hydrotreated distillate was fractioned by means of a true boiling point distillation apparatus to yield a diesel fraction in the boiling range 220° C. to 340° C. This fuel was found to contain less than 0.1% v/v aromatics and no detectable polyaromatic hydrocarbons.

The fuel typical quality is depicted below:

MEASURE TYPICAL PROPERTY UNIT TEST METHOD ANALYSIS Colour ASTM ASTM D156 +30 Density @ 20° C. kg/l ASTM D1298 0.796 Aromatic Content % (m/m) IP391 <1 Distillation: ASTM D86 90% (v/v) Recovery ° C. 320 FBP ° C. 340 Flash Point (P.M.cc.) ° C. ASTM D93 93 Kinematic Viscosity @ 40° C. CSt ASTM D445 2.7 Cold Filter Plugging Point ° C. IP309 <minus 45 Ash Content % (m/m) ASTM D482 <0.01 Sediment by Extraction % (m/m) ASTM D473 <0.01 Water Content % (v/v) ASTM D1744 (Mod) <0.01 Carbon Residue, Ramsbottom % (m/m) ASTM D524 0.15 (on 10% residue) Total Sulphur % (m/m) ASTM D2622 or 0.0004 ASTM D5453 Copper Corrosion (3 hrs @ 100° C.) Rating ASTM D130 Cetane Number — ASTM D613-IP41 54 Oxidation Stability mg/100 ml ASTM D2274 <0.1

The above fuel combined with it's low aromatics content, favourable emission qualities and excellent cold flow properties make it an excellent fuel for use in polluted cities (City Diesel) especially those with cold climates.

Example 2

Light olefins in the carbon range C3 to C6 originating from a the High Temperature Fischer Tropsch plant located in Mossel Bay were oligomerised over a proprietary zeolyte catalyst (COD 9) as supplied by Sud Chemie. The oligomerisation reaction took place at moderate temperatures below 280° C. and relatively high pressures of 55 bar process were used for the oligomerisation reaction to produce an oleffinic distillate with a Bromine Number of over 120 g Br/100 g sample. This distillate was hydrotreated in one step using a supported Platinum commercial catalyst (Axens LD402). The catalyst (270 cc) was loaded into a pilot plant a graded bed format and diluted with inert ceramics. The reactor pressure was maintained at 60 bar, the WABT did not exceed 230° C., the LHSV was maintained at 0.9 and a portion of the product was recycled.

The one step hydrotreated distillate was fractioned by means of a true boiling point distillation apparatus to yield a diesel fraction in the boiling range 220° C. to 340° C. This fuel was found to contain less than 0.1% v/v aromatics.

Emission testing performed on a similar fuel made from the process was found to offer substantial vehicle regulated reductions over commercial low sulphur diesel fuels. Reductions were noted for all regulated emissions, these included hydrocarbons, carbon monoxide, carbon dioxide, nitrous oxides and particulate matter.

The fuel was dosed with a commercial lubricity additive (OLI 5000) as supplied by Ethyl at a dose rate of 150 ppm v/v. This was found to be an ideal additive for sulphur free synthetically derived fuels as produce by the above process.

The absence of sulphur from these fuels enables modern vehicle exhaust aftertreatment technologies. In cases were these fuels are used in a bus equipped with a catalytic device the exhaust emissions were further reduced.

The fuel typical quality is depicted below:

PIONA composition as tested by GC-FIMS:

Parafins-Iso 65.3% mass Parafins-n  2.7% mass Monocycloparaffin's 24.3% mass Dicycloparaffin's  7.6% mass Aromatics <0.1% mass

The % branching of iso-paraffins;

-   -   methyl 60 to 70;     -   ethyl 2 to 10;     -   propyl 0.2 to 5;     -   butyl 0.1 to 5;     -   hexyl 0.1 to 2.

The NMR branching index is 0.165, 0 indicating absence of branching and 1 indicating full branching. 

1-24. (canceled)
 25. A process for the production of synthetically derived diesel, comprising the steps of: catalytic conversion of Fisher-Tropsch derived light olefins to distillates over a shape selective zeolite type COD-9 catalyst at a pressure of more than 50 barg and at a reactor temperature of below 280° C., whereby a COD product is obtained; one step hydrotreating the COD product to simultaneously hydrogenate both olefins and aromatics; and collecting a hydrotreated fraction boiling from about 180° C. to 360° C.
 26. A process as claimed in claim 25, wherein the one step hydrotreating step comprises hydrogenation over a Group 10 metal catalyst.
 27. A process as claimed in claim 26, wherein the Group 10 metal catalyst has a high nickel content.
 28. A process as claimed in claim 27, wherein the Group 10 catalyst comprises a noble metal supported on a carrier.
 29. A process as claimed in claim 28, wherein the noble metal is platinum.
 30. A process as claimed in claim 26, wherein the hydrotreating catalyst is bimetallic.
 31. A process as claimed claim 25, wherein a reactor pressure for the hydrotreating step reactions is from 5000 kPa to about 8000 kPa, wherein a reaction temperature for the hydrotreating step is from about 200° C. to 260° C., and wherein a liquid hourly space velocity for the hydrotreating step is from 0.3 to
 2. 32. A process as claimed in claim 26, wherein the hydrotreating catalyst is loaded into a reactor bed in an increased graded approach, whereby an excessive exothermic reaction developing at the top of the reactor is limited.
 33. A process as claimed in claim 25, further comprising a step of blending the COD product or the hydrotreated fraction with at least one component selected from the group consisting of crude derived diesel, biodiesel, and alcohols.
 34. A synthetically derived diesel produced by a catalytic conversion of Fisher-Tropsch derived olefins to distillates (COD) and one step hydrotreating thereof, the diesel boiling in the range of about 180 to 360° C., and comprising: less than 10% n-paraffins; more than 60% iso-paraffins, wherein the isoparaffins are predominantly methyl branched iso-paraffins; and less than 1% aromatics.
 35. A synthetically derived diesel as claimed in claim 34, wherein the iso-paraffins comprise from 50 to 80% methyl branched iso-paraffins.
 36. A synthetically derived diesel as claimed in claim 35, wherein iso-paraffins comprise from 60 to 70% methyl branched iso-paraffins.
 37. A synthetically derived diesel as claimed in claim 34, comprising about 20% naphthenes.
 38. A synthetically derived diesel as claimed in claim 37, wherein the naphthenes comprise predominantly monocyclic naphthenes.
 39. A synthetically derived diesel as claimed in claim 34, having a boiling range of from about 210 to 345° C.
 40. A synthetically derived diesel as claimed in claim 34, having a flash point at least as high as high as 100° C., a kinematic viscosity of from 2.2 to 2.8 cSt as tested by ASTM D445 at 40° C., a total sulfur content of the diesel below 2 ppm (m/m) as measured by ASTM 3120, a Bromine Number of below 1 mg/10 g as measured by IP 129, and a peroxide number of less than 1 mg/100 g as measured by ASTM D3703.
 41. A synthetically derived diesel as claimed in claim 40, which is a blend with at least one component selected from the group consisting of crude derived diesels, biodiesel, and alcohols. 